Annually, about 500,000 cancer-related deaths are estimated in the United States alone. Of these, approximately 13,000 are attributed to head-and-neck including oral squamous-cell carcinoma (HNOSCC), making it the sixth most common cause of cancer deaths and the fourth most prevalent cancer in men worldwide (1).
A lack of biomarkers for early detection and risk assessment is clearly reflected by the fact that more than 50% of all patients with head-and-neck squamous-cell carcinoma (HNSCC) have advanced disease at the time of diagnosis (2). The five-year survival rate of HNSCC patients is less than 50%, and the prognosis of advanced HNSCC cases has not changed much over the past three decades, except in a few advanced centers (2). Conceivably, improvement in understanding of the steps leading to tumorigenesis will provide the ability to identify and predict malignant progression at an earlier stage of HNSCC lesions, in turn leading to more effective treatment and reduction of morbidity and mortality.
The precancerous lesions, potentially malignant lesions, premalignant lesions, and squamous intraepithelial lesions (SILs) of the head and neck (oral cavity, oropharynx, and larynx)—which are clinically usually defined as “leukoplakia”—remain the main controversial topic in head and neck pathology as regards classification, histological diagnosis, and treatment (3-5). The transition from a normal epithelium to squamous cell carcinoma (SCC) of the head and neck is a lengthy, comprehensive and multistage process, causally related to progressive accumulation of genetic changes leading to the selection of a clonal population of transformed epithelial cells (6). The whole spectrum of histological changes occurring in this process has been recently cumulatively designated potentially malignant lesions or SILs, ranging from squamous hyperplasia to carcinoma in situ (CIS) (3). In their evolution, some cases of potentially malignant lesions and SILs are self-limiting and reversible, some persist, and some progress to SCC in spite of careful follow-up and treatment.
Oral squamous-cell carcinoma (OSCC), the most common form of HNOSCC, is often preceded by clinically-well-defined lesions, such as leukoplakia, causally linked with chronic exposure of the oral mucosa to carcinogens or growth promoters in tobacco and alcohol; leukoplakias with dysplasia are termed “oral premalignant lesions” (OPLs) (3, 6). The presence of dysplastic areas in the oral epithelium is associated with a likely progression to cancer; however, it is not an accurate predictor of cancer risk (6, 7). The major challenge in oral tumorigenesis is the identification of proteins that may serve as markers to differentiate the high-risk leukoplakic lesions from more benign lesions for early intervention to reduce the morbidity associated with this devastating disease. Rapid advances in treatment modalities and improvements in the early detection of head-and-neck cancers have not significantly impacted the overall survival rates of cancer patients.
Currently, there are no clinically-established biomarkers to facilitate the diagnosis or prognosis of head-and-neck cancer and oral leukoplakia. It is expected that identification of novel protein markers or therapeutic targets will ultimately improve patient care and survival. Thus, much effort has been focused on genomics- and proteomics-based identification of biomarkers that can detect the disease in early stages, predict the risk of malignant transformation in patients with oral leukoplakia, and/or predict the clinical outcome in HNOSCC patients after treatment of primary tumors. It is hoped that these biomarkers will transform clinical practice by including cancer screening and diagnosis based on molecular markers as a complement to histopathology.
In the post-genomics era, proteomics combined with mass spectrometry (MS) has become a powerful paradigm for the examination of proteins in a global manner, and the consequent discovery of cancer risk markers and drug targets. While transcriptomics provides a tool for unraveling gene-expression networks, proteomics links these networks to protein products and provides further insight into post-translational modifications that regulate cellular functions, thereby complementing genomic analyses (reviewed in Ralhan (8)). Identification of differentially expressed proteins in HNSCCs using proteomics revealed that expression patterns of proteins may have some predictive power for clinical outcome and personalized risk assessment (8-16)
Differential tagging with isotopic reagents, such as isotope-coded affinity tags (ICAT) (17) or the more recent variation that uses isobaric tagging reagents, iTRAQ (Applied Biosystems, Foster City, Calif.), followed by multidimensional liquid chromatography (LC) and tandem mass spectrometry (MS/MS) analysis, is emerging as one of the more powerful methodologies in the search for disease biomarkers. Recent studies using iTRAQ reagents resulted in identification and relative quantification of proteins leading to a discovery of potential cancer markers (PCMs) for human cancers (17-23).